This invention relates to techniques for scrambling and descrambling audio information signals. More particularly, this invention relates to frequency shifting techniques for scrambling and descrambling such signals.
Various techniques have been employed in the past for the purpose of initially scrambling and subsequently descrambling audio information signals. One such technique is known as frequency inversion spectrum shifting, wherein the spectrum of original audio information signals is shifted by inversion so that those frequency portions originally lying at the lower end of the audio frequency band are shifted to the upper end while those portions originally lying near the upper end of the band are shifted to the lower end. Typically, this spectral inversion of the original audio information signals is performed prior to broadcasting or recording the signals either alone, or in combination with associated video signals, and this technique is described more fully in copending, commonly assigned U.S. patent application Ser. No. 366,575, filed Jun. 15, 1989 for "IMPROVED METHOD AND SYSTEM FOR SCRAMBLING AND DESCRAMBLING AUDIO INFORMATION SIGNALS", now U.S. Pat. No. 5,058,159 issued Oct. 15, 1992, the disclosure of which is hereby incorporated by reference. The major purpose of such scrambling is to prevent unauthorized reception or reproduction of the signals. As one commercial example, pre-recorded video cassettes can be rendered unintelligible by scrambling the audio information portion, so that only an authorized subscriber having a proper descrambling unit coupled to the television monitor/receiver can enjoy the program information by descrambling the audio portion.
A major disadvantage with known audio scrambling devices using frequency inversion spectrum shifting techniques is the introduction of frequency error upon recording and reproduction of the scrambled signals, which adversely affects the descrambling process. In particular, even if the scrambled audio signals are recorded on a cassette tape in a high quality VCR, the amount of frequency error introduced as wavering is at least .+-.1% at the carrier frequency for a typical unit. This frequency error introduces unwanted components into the recovered signals, resulting in garbled sounds which are annoying at best and unrecognizable at worst.
As a specific example, with a frequency spectrum inversion technique employed for audio signals having a bandwidth of 15 KHz, the typical minimum carrier frequency would be 1 KHz above the upper end of the band, or 16 KHz at base band. If a 700 Hz audio signal is scrambled using the 16 KHz carrier and then recorded on a tape in a quality VCR, the recorded signal will be 16 KHz-700 Hz.+-.1%=15.3 KHz.+-.153 Hz. Upon reproduction and descrambling using a carrier of the same frequency, the descrambled signal will be 16 KHz-[15.3 KHz.+-.153 Hz]=700 Hz.+-.153]Hz=700 Hz.+-.21.9%. As will be appreciated by those skilled in the art, such a wide amount of wavering distortion will render the descrambled audio signals at least unpleasant if not unintelligible.
Another disadvantage encountered with frequency inversion techniques in audio signal processing results from the pre-emphasis signal processing normally encountered in broadcasting environments and in many recorders. More particularly, in a broadcasting application pre-emphasis is applied in an amount of +6 db per octave beginning at about 2 KHz. For audio signal processing, since the upper edge of the bandwidth (i.e., 15 KHz) lies in an area of high pre-emphasis, and since much of the audio energy upon frequency inversion is located at the upper edge, only relatively low signal levels can be appropriately input to a broadcast or recording system, which reduces the signal to-noise ratio by an undesirable amount.